(a) Field of the Invention
This invention relates to carburetors and a control for governing the richness of the fuel mixture being fed to an associated engine upon start-up. More particularly, it resides in an arrangement of air passages within a carburetor that reduce the pressure drop between the fuel bowl and the carburetor throat to lean out the fuel mixture immediately after initial starting to prevent the engine from stalling, and it has particular application to relatively simple carburetors for small gasoline engines in which the carburetor choke and throttle valves are manually adjusted.
(b) Description of the Art
Conventional carburetors are characterized by a choke valve located in the carburetor throat on the upstream side of a venturi constriction and a throttle valve on the downstream side of the venturi. When a cold engine is to be started the choke valve is closed to develop a pressure drop between the fuel bowl and the venturi constriction in response to engine suction. The relatively high pressure in the fuel bowl causes a substantial fuel flow to the carburetor venturi to develop a rich fuel-to-air mixture enabling the engine to fire. For quick starting the choke valve should be tight, or "severe", with a minimum of air leakage through or around its air blocking surface in order to obtain a relatively high pressure drop between the fuel bowl and the venturi. However, once the engine commences running the pressure in the carburetor throat tends to drop due to greater engine demand, and with a tight choke a greater pressure differential then develops between the fuel bowl and venturi. This increased pressure differential may cause the fuel to air mixture to become too rich with a consequent stalling of the engine.
In order to reduce the richness of the fuel mixture after initial starting, some choke valves have been equipped with mechanical spring loaded relief valves mounted on their surfaces which automatically open in response to increased engine air demand, thus allowing air flow through the choke valve even though it is in closed position. The increased air flow is to satisfy engine demand and also reduce the richness of the fuel-air mixture. Such designs have not been entirely satisfactory because they have an inherent problem of spring rate sensitivity, and are subject to sticking due to corrosion, accumulations of foreign matter and ice build-up in cold weather.
Another approach to leaning out the fuel mixture has been to employ small relief holes in the surface of the choke valve, or to provide small bleeder holes around the valve. Such designs have limitations in that they are a compromise between the best configurations for cranking and initial running. Also, during very cold weather, approximately 0.degree. F. or below, a carburetor with relief holes for the choke valve must have the throttle valve set in a near idle, or closed, position to create a lower pressure on the throttle valve downstream side to sufficiently vaporize the fuel for starting. However, in very cold weather it is desirable to set the throttle valve in a more open position, such as fast idle or wide open to deliver a greater amount of vaporized fuel for combustion. Sufficient vaporized fuel needed during cold weather may not be developed with an open throttle valve unless a very severe choke valve is used. However, if a severe choke is used, a fuel mixture too rich for combustion may result after initial engine start. The present invention has been developed to answer the problem of controlling engine fuel mixtures during cranking and initial firing, and for continued operation until the choke valve is adjusted.